Investments work like this: the best edge available for the user is the one whose bankroll will be used. So for example, if there is an investment at 99.5% (i.e. 0.5% house edge) and nothing higher, then that will be offered to players

You participate based on your percentage of the bankroll. In theory though, larger bankroll means more players and allowing larger bets so return should be similar regardless of how much you invest.

How to choose which edge to invest in?

Your investment stays at the fixed edge you invest in. So if you invest at 3.5% (what we call 96.5), only games played at that level will use your funds. You can also invest at multiple edges and you can move investments between edges.

So generally speaking your investment will be used to gamble with a positive edge.

There are many strategies - for me personally I would invest some at the highest funded Edge to get some action and invest some at other edge (better house edge) for if those funds at current playable edge are closed out.